This invention relates generally to optical communication. More particularly, it relates to optical switches for wavelength division multiplexing.
Optical wavelength division multiplexing (WDM) has gradually become the standard backbone network for fiber optic communication systems. WDM systems employ signals consisting of a number of different wavelength optical signals, known as carrier signals or channels, to transmit information on optical fibers. Each carrier signal is modulated by one or more information signals. As a result, a significant number of information signals may be transmitted over a single optical fiber using WDM technology.
WDM systems use components referred to generically as optical interleavers to combine, split, or route optical signals of different channels. Interleavers typically fall into one of three categories, multiplexers, de-multiplexers and routers. A multiplexer takes optical signals of different channels from two or more different input ports and combines them so that they may be coupled to an output port for transmission over a single optical fiber. A de-multiplexer divides a signal containing two or more different channels according to their wavelength ranges and directs each channel to a different dedicated fiber. An optical interleaver can spatially separate dense WDM (DWDM) or ultra-dense WDM (UWDM) signals into two complementary subsets, each having twice the original channel spacing. A router works much the same way as a de-multiplexer. However a router can selectively direct each channel according to control signals to a desired coupling between an input channel and an output port.
FIG. 1 depicts a typical optical interleaver 999 of the prior art as described in U.S. Pat. No. 5,694,233, issued to Wu et al. on Dec. 2, 1997. A WDM signal 90 containing two different channels 91, 92 enters interleaver 999 at an input port 11. A first birefringent element 30 spatially separates WDM signal 90 into horizontal and vertically polarized components 101 and 102 by a horizontal walk-off. Each of component signals 101 and 102 carries the full frequency spectrum of WDM signal 90.
Components 101 and 102 couple to a polarization rotator 40, which selectively rotates the polarization state of either signal 101 or 102 by a predefined amount. For example, rotator 40 rotates signal 102 by 90xc2x0 so that signals 103, 104 are both horizontally polarized as they exit rotator 40 and enter a wavelength filter 61.
Wavelength filter 61 selectively rotates the polarization of wavelengths in either the first or second channel to produce filtered signals 105 and 106. For example wavelength filter 61 rotates wavelengths in the first channel 91 by 90xc2x0 but not the second channel 92. The filtered signals 105 and 106 enter a second birefringent element 50 that vertically walks off the first channel into beams 107, 108. The second channel forms beams 109, 110.
A second wavelength filter 62 then selectively rotates the polarizations of signals 107, 108 but not signals 109, 110 thereby producing signals 111, 112, 113, 114, having polarizations that are parallel each other. A second polarization rotator 41 then rotates the polarizations of signals 111 and 113, but not 112 and 114. The resulting signals 115, 116, 117, and 118 then enter a third birefringent element 70. Second wavelength filter 62 may alternatively be replaced by a polarization rotator 41 suitably configured to rotate the polarizations of signals 111, 113 but not 112, 114.
Third birefringent element 70 combines signals 115 and 116, into the first channel, which is coupled to output port 14. Birefringent element 70 also combines signals 117 and 118 into the second channel, which is coupled into output port 13.
As described above, interleaver 999 operates as a de-multiplexer. By operating interleaver 999 in reverse, i.e., starting with channels 91, 92 at ports 13 and 14 respectively, interleaver operates as a multiplexer. Furthermore, by suitably controlling the polarization rotation induced by rotators 40 and 41, interleaver 999 may be configured to operate as a router.
Interleaver 999 has certain drawbacks. First, each port requires its own collimator. Three collimators take up space and require a relatively large walk-off distance for the signals. Consequently, birefringent elements 30, 50 and 70 tend to be both long and wide. Second, the number of components, particularly birefringent elements, tends to make interleaver 999 bulky, expensive and more massive. Generally, as the mass of interleaver 999 increases, its operation becomes more unstable. Third, the coupling distance, i.e., the distance between port 11 and ports 13, 14, tends to be long, which increases insertion losses in interleaver 999. Furthermore, each of the ports 11, 13 and 14 requires a separate collimator to couple the signals into and out of optical fibers. This adds to the complexity and expense of interleaver 999.
Passband wavelength accuracy and channel isolation are important in DWDM and UWDM applications. For accurate passband wavelength, typically 1% of channel spacing, it is necessary to produce wavelength filters with a length that is accurate to within 1% of the wavelength. Furthermore, the angle between the optic axis of the filter and the polarization of the signal must be carefully controlled. These two requirements increase the cost of current interleavers.
Using a folding design can reduce the complexity and expense of an interleaver. A folding design follows the same general principles as interleaver 999 but uses only two birefringent crystals instead of three. A reflector coupled to the second birefringent crystal reflects the light back through the interleaver elements along a reverse path. Thus, the interleaver elements are fewer and can be made smaller, thereby saving space, complexity and cost.
Unfortunately, even folding designs have drawbacks. One of these drawbacks is that in order to assure proper multiplexing, demultiplexing, or routing, beams containing light in the same wavelength range must be on the same xe2x80x9csidexe2x80x9d of the interleaver to be properly combined. The prior art uses complex polarization rotation schemes and wavelength filters to accomplish this. Unfortunately, these rotation schemes and wavelength filters add to the manufacturing cost of the interleaver and introduce additional insertion losses.
There is a need, therefore, for an improved optical interleaver that overcomes the above difficulties.
Accordingly, it is a primary object of the present invention to provide a folded optical interleaver that uses fewer and/or smaller birefringent elements than in previous designs. It is a further object of the invention to provide a folded interleaver having a routing capability implemented by a single polarization rotation element.
The above objects and advantages are attained by an inventive optical interleaver apparatus and method. The apparatus comprises a first birefringent element, a first waveplate, a second birefringent element, a polarization rotation device and a retro-reflector. An optic axis of the first birefringent element is oriented such that the first birefringent element separates an input optical signal into first and second beams having complementary polarizations. The first waveplate is optically coupled to the first birefringent element. The first waveplate rotates a polarization of light in a first wavelength range, but not a second wavelength range. The second birefringent element is optically coupled to the first waveplate. The second birefringent element separates the first beam into a third beam containing light in the first wavelength range and a fourth beam containing light in the second wavelength range. The third beam and the fourth beam have complementary polarizations. The second birefringent element also separates the second beam into a fifth beam containing light in the second wavelength range and a sixth beam containing light in the first wavelength range. The fifth beam and the sixth beam have complementary polarizations but the polarizations of the third and fifth beams are substantially the same as each other and the polarizations of the fourth and sixth beams are substantially the same as each other. The polarization rotation device is optically coupled to the second birefringent element. The polarization rotation device rotates the polarization of selected beams of the third, fourth, fifth and sixth beams so that they are properly recombined by the second birefringent element on a return path. The polarization rotation device may be a second waveplate or a non-reciprocal rotator.
The retro-reflector, which is optically coupled to the polarization rotation device, reflects the third, fourth fifth and sixth beams traveling along forward paths from the polarization rotation device back towards the polarization rotator and the second birefringent element along reverse paths. On the reverse path the second birefringent element combines the third beam with the sixth beam to form a first output optical signal containing light in the first wavelength range. The second birefringent element also combines the fourth beam with the fifth beam to form a second output optical signal containing light in the second wavelength range.
The inventive apparatus may operate as either a demultiplexer or multiplexer. In an alternative embodiment, the apparatus includes a switchable polarization rotator disposed between the first birefringent element and the first waveplate. The switchable polarization rotator allows the apparatus to operate as a router by either rotating or not rotating the polarizations of the first and second beams.